Today's hearing aids include only one receiver that, together with the hearing-aid acoustics (tubing, wax protection devices, etc.) connected to it, has a resonance frequency that lies between 2 kHz and 3.5 kHz. There are two primary reasons for this limitation. First, the un-occluded ear has significant gain in this frequency range, which is removed by blocking the open ear canal with an closed-fitting earmold. Second, in order to achieve an acceptable output and efficiency at both low and high frequencies, the resonance frequency is selected to be somewhere in the middle of the required frequency range (e.g., 300 Hz to 6 kHz). If the resonance frequency is increased above 3.5 kHz, the efficiency would be too low for the low frequencies though it would improve the response above 4 kHz considerably.
There is a trend to increase the bandwidth of the hearing aid, but this trend is particularly difficult to apply to behind-the-ear (BTE) hearing aids because the long sound tubing inserted between the receiver sound port and the sound outlet of the ear mold suppresses the high frequencies. Bandwidth enhancement in general has been limited by the available processing power of the DSPs within the hearing aid, in which the audio sampling rates typically have been limited to a sample rate of about 16 kHz with a resulting audio bandwidth slightly below 8 kHz. In the increasingly popular open-fitting “over-the-ear” (OTE) hearing aids, overall performance with respect to frequency bandwidth and efficiency can be improved by placing the receiver deeper inside the user's ear canal.
Thus, a need exists for improved hearing aids that will amplify and output substantial sound pressure in the frequency range above 8 kHz in addition to the ordinary sound pressure output in the frequency range 100 Hz to 8 kHz. The present invention is directed to satisfying one or more of these needs and solving other problems.